lab 3: adc

Network and Systems Laboratorynslab.ee.ntu.edu.tw


Signal Path

Sensors Signal Conditioning

(if necessary)

Convert to voltage

• Amplification• Filtering

• Light• Temperature• Acceleration• Humidity• Pressure• etc.

• Resistance• Capacitance• Current• Voltage• etc.

Analog to Digital

Conversion

(ADC)

10011101

Produce a proper output

voltage level

Converts voltage to

digital numberToday’s

labs


General ViewPositive/Negative voltage reference: define the upper and

lower limits of the conversion

12 input channels available

Software configurable

sample-and-hold period

16 conversion result storage

registers

Each storage register

associated with a control register

4 clock sources available for ADC12 clock


Conversion FormulaThis is a conversion formula of ADC

For MSP430 ADC12 moduleIt is 12-bit ADC

If Vin > Vref+ => output 4095 (max)If Vin < Vref- => output 0 (min)

refref

refbitnADC VV

VVinN )12(

refref

refADC VV

VVinN )4095(


Input Channels12 input channels available

8 external input pins: A0 ~ A72 external voltage reference pins: VeREF+, VeREF-1 internal temperature sensor1 supply voltage monitor

Multiplexed with P6 GPIOSetting P6SEL to choose the

function you want


Internal Voltage Reference GeneratorVoltage references are used as a base of the

conversionInput voltage compares to the voltage reference to

generate the conversion result

A stable voltage reference is crucial to an accurate conversion Usually we use a special circuit to generate stable

voltage referenceMSP430 has a built-in voltage reference generator

It has two selectable voltage levels 1.5V and 2.5V

refref

refbitnADC VV

VVinN )12(


Voltage References Options3 positive reference sources

AVcc – Positive supply voltageVREF – internal voltage reference

Can be 1.5V or 2.5V, depends on your selection

VeREF+ – external positive voltage reference

2 negative reference sourcesAVss – Negative supply voltage (GND)VeREF- -- external negative voltage reference

6 combinations in total


ADC12 Conversion ModesSingle channel single-conversion

A single channel is converted onceSequence-of-channels

A sequence of channels is converted onceRepeat-single-channel

A single channel is converted repeatedlyRepeat-sequence-of-channels

A sequence of channels is converted repeatedly


Conversion Memory16 ADC12MEMx memory registers to store

conversion resultsEach ADC12MEMx is configured with an

associated ADC12MCTLx control register.EOS: End of sequence, indicates last conversionSREFx: Select voltage referenceINCHx: Select Input Channel

CSTARTADDx bits define the first memory used for conversionThis bit is in ADC12CTL1 control register


ADC12MCTLx

Use in sequence of

channels mode


ExampleDepends on the setting of conversion mode,

ADC12MCTLx and CSTARTADDx, the ADC will have different behavior

Single-channel

CSTARTADDx = 0000

You need to set ADC12MCTL0 (choose the input channel and voltage reference). And the conversion result will store in ADC12MEM0

CSTARTADDx = 0100

You need to set ADC12MCTL8 (choose the input channel and voltage reference). And the conversion result will store in ADC12MEM8

Sequence of Channels

CSTARTADDx = 0000

You need to set ADC12MCTL0 to ADC12MCTL4 (choose the input channel and voltage reference for each ADC12MEM). You need to set the EOS bit in ADC12MCTL4 to 1, indicates the sequence of conversion end here. And the conversion results will store in corresponding ADC12 memory registers

CSTARTADDx = 0100

If you need 5 input channels

If you need 7 input channels

You need to set ADC12MCTL8 to ADC12MCTL14 (choose the input channel and voltage reference for each ADC12MEM). You need to set the EOS bit in ADC12MCTL14 to 1, indicates the sequence of conversion end here. And the conversion results will store in corresponding ADC12 memory registers


Conversion Clock4 clock sources available

ADC12OSC: ADC12 internal oscillatorMCLKSMCLKACLKFor simplicity, we use ADC12OSC in our labIndependent of the system clock


ADC12 CoreTwo phases for a conversion

Sample-and-holdA/D conversion

Conversions in MSP430 ADC12Triggered by a SHI signalTwo sample modes

Extended Sample Mode

Pulse Sample Mode

SHI signal triggers a conversion and controls the length of sample-and-hold

period

SHI signal triggers a conversion, the length of the

sample-and-hold period is controlled by other register


The SHI SignalFour selectable sources for SHI

ADC12SC bit A single bit in ADC12 control register, set this bit to

high will trigger a conversionTimer_A output unit 1 (TA1)Timer_B output unit 0 (TB0)Timer_B output unit 1 (TB1)

Lab_2 slide


Sample And Hold Revisit

There is a sample and hold circuit before A/D conversion

Mostly integrated with the ADC chip When no conversion, switch S1 is open When a conversion start

S1 closed Input signal charge C1 S1 open, C1 holds the value of input

signal A/D conversion

Sample and hold time Time between S1 close and re open If it is too short

C1 will not fully charged (error)


Sample Timing Considerations

For an accurate conversion, the sample-and-hold time must be long enough to charge up CI to Vs

Rs will affect the sample-and-hold time (tsample)For MSP430F1611, RI = 2K ohm (max); CI = 40

pF (max)Minimum tsample for 12-bit conversion

=>


Setting tsample

If we use ADC internal oscillator (ADC12OSC) as

ADC12 clock source. Frequency of ADC12OSC is about 5MHz, if you set SHT0x to 0100, then the sample-and-hold time is

64*(1/5000000) second = 12.8 μs


Rs: Source ResistanceMany devices didn’t specify their source

resistance (source impedance/output impedance)

Measuring source impedance is not an easy task

For most of the case, set the SHTx bit to 0001 or 0010 should work

My suggestion isit’s only 16 possible setting, just try it out!


ADC12 InterruptsADC12 has 18 interrupt sources:

ADC12IFG0-ADC12IFG15 flag Associated with each ADC12MEMx, indicates a

conversion of this memory is completeADC12OV: ADC12MEMx overflow

A conversion result is written to any ADC12MEMx before its previous conversion result was read

ADC12TOV: conversion time overflow Another conversion is requested before the current

conversion is completed


ADC12IV, Interrupt Vector GeneratorAll ADC12 interrupts are source to a single

interrupt vectorSimilar to Timer_A3

The highest priority enabled ADC12 interrupt generates a number in the ADC12IV registerHighest priority interrupt generates a number

Branch to the ADC ISR, read ADC12IV, process the associate sub-routine, leave the ADC ISR

And then, second highest priority interrupt generates another number Branch to the ADC ISR again, ……….


Reset of InterruptsOverflow interrupts (memory overflow and

conversion time overflow) will reset automatically whenAny access, read or write, of the ADC12IV

registerADC12IFG0-ADC12IFG15 flag will reset

whenAccessing their associated ADC12MEMx

register


Interrupt Enable Register

Enable the interrupt of corresponding ADC12 memory (ADC12MEM).

Usually, you only need to enable one of the source.


ADC12 Configuration


ADC12 ConfigurationEnable/disable internal voltage

reference generator, set the voltage

reference to 1.5/2.5 V

Enable/disable overflow interrupt


ADC12 Configuration

We use this bit in ADC12CTL0 register to trigger a conversion. You

can try out the other source of trigger. The benefit of using Timer_A

or Timer_B trigger sources is you don’t need CPU to handle the trigger


ADC12 Configuration

We use ADC12 internal

oscillator as clock source

Depends on your requirement, choose

the proper conversion sequence

mode

Use Pulse Sample Mode


Example: Single Channel, single conversionOnly one input channel, one conversion at a

timeInitialization

Turn-on ADC12

Enable Conversion

Configure the ADC: ADC12CTL0 1. setting SHTx 2. setting voltage reference ADC12CTL1 1. setting CSTARTADDx (pointed to one memory) 2. setting SHSx 3. SHP = 1 4. setting ADC12 clock source 5. select single channel, single conversion modeADC12MCTLx 1. depends on the conversion mode you choose, and the input channel and voltage reference you want, set proper ADC12MCTLx register

Set ADC12ON to 1

Set ENC to 1Trigger a Conversion

You can use a timer to trigger a conversion, in the timer ISR, set

the ADC12SC bit to 1

An ADC interrupt generated when the conversion complete. The conversion result is available at the memory register you choose

You need to enable the corresponding interrupt.

The one selected by CSTARTADDx


Example: Sequence of ChannelsMultiple input channels, a sequence of

conversionsInitialization

Turn-on ADC12

Enable Conversion

Configure the ADC: ADC12CTL0 1. setting SHTx 2. setting voltage reference 3. set MSC bit to 1 (multiple conversion automatically start) ADC12CTL1 1. setting CSTARTADDx (pointed to one memory) 2. setting SHSx 3. SHP = 1 4. setting ADC12 clock source 5. select Sequence-of-channels modeADC12MCTLx 1. depends on the conversion mode you choose, and the input channel and voltage reference you want, set proper ADC12MCTLx register 2. depends on the number of channels you want, set the EOS bit to 1 in the proper ADC12MCTLx register

Set ADC12ON to 1

Set ENC to 1Trigger a Conversion

You can use a timer to trigger a conversion, in the timer ISR, set

the ADC12SC bit to 1

An ADC interrupt generated when the last conversion complete. The conversion results are available at the memory registers you choose

You need to enable the interrupt of the last conversion memory.


Today’s LabsLab 1: Light Sensor

use the sample code on websitechoose a light sensor on Tarokoconfigure the ADC to take samples from the

light sensorwhen the light sensor is covered by hand, turn

on a LED; when the hand remove, turn off the LED set a threshold


Lab2: Infrared Proximity SensorConnect the sensor to

Tarokoconfigure the ADC to take

samples from the sensoruse an obstacle(hand, paper,

etc.) to approach the sensorif distance < 10 cm, Red

LED onif distance > 20 cm, Green

LED onelse, Yellow LED on

VccGND

Signal


Lab 3: AccelerometerConnect the accelerometer module to TarokoConfigure a GPIO pin as a supply voltage source

for the accelerometer moduleconfigure the ADC to take samples from multiple

input channelsConfigure the LEDs

when accelerate in +X direction, turn on Red LED; accelerate in -X direction, turn off Red LED

when accelerate in +Y direction, turn on Green LED; accelerate in -Y direction, turn off Green LED

when accelerate in +Z direction, turn on Yellow LED; accelerate in -Z direction, turn off Yellow LED

lab 3: adc

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